(1) Field of the Invention.
The invention relates to a transmission system for transmitting a multicarrier signal from a transmitter to a receiver.
The invention further relates to a receiver for receiving a multicarrier signal from a transmitter, an equalizer for canceling intercarrier interference included in a multicarrier signal, and to a method of canceling intercarrier interference included in a multicarrier signal.
(2) Background Art.
Multicarrier signal modulation methods, such as OFDM and MC-CDMA, have been around for some time now. OFDM or Orthogonal Frequency Division Multiplexing is a modulation method designed in the 1970's in which multiple user symbols are transmitted in parallel using different subcarriers. These subcarriers have overlapping (sinc-shaped) spectra, nonetheless the signal waveforms are orthogonal. Compared to modulation methods such as BPSK, QPSK or MSK, OFDM transmits symbols which have a relatively long time duration, but a narrow bandwidth. Mostly, OFDM systems are designed such that each subcarrier is small enough in bandwidth to experience frequency-flat fading. This also ensures that the subcarriers remain orthogonal when received over a (moderately) frequency selective but time-invariant channel. If the OFDM signal is received over such a channel, each subcarrier experiences a different attenuation, but no dispersion.
The above mentioned properties of OFDM avoid the need for a tapped delay line equalizer and have been a prime motivation to use OFDM modulation methods in several standards, such as Digital Audio Broadcasting (DAB), the Digital Terrestrial Television Broadcast (DTTB) which is part of the Digital Video Broadcasting standard (DVB), and more recently the wireless local area network standard HIPERLAN/2. Particularly in the DAB and DTTB applications, mobile reception under disadvantageous channel conditions are foreseen, with both frequency and time dispersion. Mobile reception of television has not been regarded as a major market up to now. Nonetheless, the DVB system promises to become a high-speed delivery mechanism for mobile multimedia and internet services. At the IFA '99 Consumer Electronics trade show, a consortium of Nokia, Deutsche Telecom and ZDF demonstrated mobile web browsing, email access and television viewing over an OFDM DVB link, with a GSM return channel. With 8k OFDM subcarriers, over the air DVB reception functioned properly for vehicle speeds up to 50 mph. Mobile reception, i.e. reception over channels with Doppler spreads and the corresponding time dispersion remains one of the problems associated with OFDM systems in particular and multicarrier transmission systems in general. Whereas its robustness against frequency selectivity is seen as an advantage of OFDM, the time-varying character of the channel is known to limit the system performance. Time variations are known to corrupt the orthogonality of the OFDM subcarrier waveforms. In such a case, Intercarrier Interference (ICI, also referred to as interchannel interference or FFT leakage) occurs because signal components from one subcarrier cause interference to other, mostly neighboring, subcarriers.
In the paper “An Equalization Technique for Orthogonal Frequency-Division Multiplexing Systems in Time-Variant Multipath Channels”, IEEE transactions on Communications, Vol. 47, No. 1, January 1999, pages 27–32, a multicarrier transmission system is disclosed. In this known transmission system ICI is cancelled (i.e. detected and removed from the received multicarrier signal) in the receiver by means of an equalizer. This equalizer comprises a digital filter which derives a vector of estimated symbols from a vector of received symbols. The filter in the known transmission system is relatively complex, i.e. a relatively large number of computations is needed to implement the filter.